Organosiloxane elastomers



Aug. 30, 1949. A m v 2,480,620

ORGANOS ILOXANE ELASTOMERS Filed Dec. 27, 1946 WNW/r56 250 [mu [$477150MoRR/c/r I? FIFMA unmen .50 mo /50 200 6406: J TEAM 0oz.

Patented Aug. 30, 1949 ORGANOSILQXANE ELASTOMERS Earl Leathen Warrick,Pittsburgh, Pa., asslgnor to Corning Glass Wig-ks, Corning, N. Y., acorporation of New Yor- Application December 27, 1946, Serial No.718,857 6 Claims. (Cl. 260-465) The present invention relates to thefabrication of elastomers of the organosiloxane type and particularly tomethods for use in the vulcanization thereof.

The organosiloxanes are polymers constituted of polymer units of thetype RnsiO4-n, wherein R is a monovalent organic radical bonded to thesilicon by a carbon-silicon linkage. The organo polysiloxanes are ofdiverse physical form and utility. Their physical form and resultantutility is dependent to a large extent upon the value of the integer nin the siloxane units contained in the polymer and the relativedistribution of these types of units. The physical characteristics arelikewise, but generally to a more limited extent, dependent upon thespecific organic radicals represented by R. By variation of thesefactors, products are obtained which are low molecular weight compoundsand have determinable melting and boiling points, which are of highaverage molecular weight and are thermosetting or thermoplastic resinouscompositions and which are of molecular weight varying from low to highand are stable fluids.

In my copending patent applications Ser. Nos. 557,055 and 557,056, bothfiled October 3, 1944; Ser. No. 607,101, filed March 1, 1946; Ser. Nos.651,382 and 651,383, both filed March 1, 1946; and Ser. No. 655,245,filed March 18, 1946, it i shown that elastomeric products areobtainable in the organosiliconclass of materials. Application, SerialNo. 557,056, has now become Patent No. 2,460,795. These products areprepared by the vulcanization of cream polysiloxanes which have anaverage degree of substitution between 1.75 and 2.25 and which containat least 40 mol per cent of diorganosiloxane units of the typ RRSiO inwhich R represents alkyl and R represents alkyl or aryl. The alkylradicals preferably contain less than 8 carbon atomsper radical. Theseorganosiloxanes from which the elastomers are produced may vary indegree of condensation from relatively low molecular weight materialsthrough and including high molecular weight materials.

vulcanization of these organosiloxanes is effected .by heating theorganosiloxane with an appropriate vulcanizing agent. Preferredvulcanizing agents are the diorgano peroxides in which at least one ofthe organic radicals is an aromatic acyl. This group .of vulcanizingagents includes alkyl aromatic-acyl peroxides, such as tertiarybutylperbenzoate, and the diacyl peroxides, such as benzoyl peroxide andacetyl bendefined class have been employed for vulcanization of theorgano-siloxanes and all have been found efiective. Vulcanizing agentsof this type are employed in amount between 1 and 10% based on theamount of siloxane present. Other suitable vulcanizing agents includediacetyl peroxide, triphenylstibine, dimethyl mercury, tributyltinacetate, tetraethyllead, tetraphenylbismuth, lead tetraacetate, mercuryacetate, CrOa, CrOa-l-CuO,

and COzOs-i-CuO.

The specific physical properties of the clastomers produced depend uponthe specific organopolysiloxane employed, its degree of polymerizationand upon the specific vulcanizing agent and filler which are used. Whileelastomers are obtainable by vulcanizing low molecular weight zoylperoxide. A wide range oi peroxides of the u siloxanes, improved tainedby physical properties are obemploying siloxanes which contain longunbranched chain structures whether the siloxane is in the form of asoluble liquid or an insoluble, tacky, rubbery solid.

Suitable fillers include asbestos, clay, silica aerogel, fiber glass,iron oxides, bentonite, alumina, zinc oxide, magnesia, lead oxide,titania, carbon black and other similar materials. 0rganic fillers, suchas cellulose, may be employed if desired, since it has been found thatthe organic fillers ,are stabilized by the siloxane.

To eflect vulcanization, the masticated and compounded organosiloxane,which includes the siloxane, the vulcanizing agent, and if desired anyfiller and mold release agent, is heated at a temperature above C. It ispreferred in order to obtain the most rapid curing that the temperatureat which the composition is heated be also above the decompositiontemperature of the peroxide in those cases in which a peroxide isemployed for vulcanization.

One of the common characteristics of the indicated materials is thatduring vulcanization, bubbles form within the material which cause theproduct to the spongy. This characteristic can be overcome readily byvulcanizing the compounded silioxanes in pressure molds.

Objects of the present invention are to provide improved methods forhandling compounded siloxanes during vulcanization; and to providemethods for vulcanizing compounded siloxanes which prevent thevulcanized product from being spongy while avoiding the use of pressuremolds during vulcanization.

In accordance with a preferred form of the present invention, thecompounded organwiloxane is vulcanized under controlled conditions incontact with saturated steam. The vulcanized .per square inch, if

CD, a weak product conducted for a period elastomer is then cured in anoven to improve the physical properties thereof.

The accompanying drawing shows the correlation of time and pressureemployed. in the vulcanization in accordance with the present invention.Steam pressure in pounds per square inch gauge is graphed horizontallyon a linear scale against time in minutes graphed vertically on athree-cycle log scale.

In the first step of the process hereof, the compounded organosiloxaneis vulcanized in direct contact with saturated steam at a gauge pressureof between and 250 pounds per square inch for a time correlated with thepressure to lie above the line AB and below the line CD in theaccompanying drawing. When operating at a gauge pressure of between 10and 50 pounds per square inch, if a time is employed, which, correlatedwith the pressure, would lie below the line AB, the vulcanizationobtained is insufllcient .to prevent the formation of bubbles in thematerial during subsequent curing. when operating at a gauge pressure ofbetween 75 and 250 pounds a time is employed which correlated withpressure would lie above the line is produced upon curing, probably dueto depolymerization as a result of the introduction of hydroxylradicals. Pressures less than 10 p. s. i. are insuflicient to preventfoaming during vulcanization. Pressures above 250 p. s. i. should not.be employed inasmuch as foaming of the organosiloxane polymer resultsdespite the high pressure. Preferred operating pressures are between 10and 150 p. s. i. For economic reasons it is preferred that thevulcanization in the presence of saturated steam be of not over 1000minutes and preferably for a period of less than 10 minutes 'but for atleast 2 minutes. In order for this first step to be entirely effectivein preventing formation of bubbles in the material, the period from thetime of exposure of the material to a temperature of 80 C. to the timethe pressure is up to 10 p. s. i. should be less than two minutes.

A preferred manner of operation involves the gradual reduction of thesteam pressure at the end of the vulcanizing step. This gradualreduction may be effected by or by stepwise reduction of the steampressure. This gradual reduction allows the gases within the vulcanizedmaterial a better opportunity to diffuse out of the material.

During the vulcanization in contact with saturated steam. the steamserves three fundamental purposes; first, it supplies heat for thevulcanization. Second, it excludes air from the elastomer. which at thisstage is detrimental. Third,

it prevents the elastomer from becoming spongy.

The minimum steam pressure of 10 p. s. 1. gauge provides a temperatureof 116 C. which is in excess of the minimum temperature forvuloanization.

In the second step of the process hereof, the vulcanized siloxane iscured at substantially atmospheric pressure at a-temperature above thatemployed in the first step. The atmosphere in contact with the siloxaneduring curing is not particularly critical under the conditions stateddue to the general inertness of the vulcanized siloxane. The siloxane isheld at the elevated temperature for at least an hour in this second orcuring step. During this curing the desirable properties of the siloxanedevelop and vary. In

. general, there is obtained an increase in the tenslow release of thesteam minute. The pressure was maintained at 20 sile strength. adecrease in elongation, and an increase in hardness. These propertieschange progressively as the curing progresses under the describedconditions. Curing conditions should therefore be selected to give thecombination of properties desired. Curing for more than 48 hours or at atemperature higher than 300 C. generally is undesirable.

In order to obtain vulcanization without bubbling in the first stehereof, it is necessary that the compounded siloxane be in directcontact with the saturated steam in some portion of its superficialsurface, though it is unnecessary that all or substantially all of thesurface be in contact therewith. Thus, the process hereof may beemployed to fabricate sheets of siloxane elastomer by vulcanizing thecompounded siloxane in a steam autoclave with the entire surface of thesheet in direct contact with the steam.

Alternatively, if it isdesired to produce a sheet with two smooth-finishsurfaces, the compounded siloxane can be placed between two plates andthe plates placed in the steam autoclave, whereby the steam is in directcontact with the compounded siloxane at the edges of the sheet, andthereby prevents volumetric expansion of the elastomer. Thevulcanization of extruded forms. such as tubing, made of theseelastomers presents a very serious problem inasmuch as the vulcanizationthereof in a press is impracticable commercially. This invention isparticularly applicable to the vulcanization of forms of this characterand produces particularly beneficial resuits.

The process hereof may be conducted in a continuous manner in thefabrication of such items as rubber covered wire. A coating of thecompounded siloxane is extruded upon the surface of the wire, and thecovered wire is drawn through a continuous steam vulcanizer and thenthrough a curing oven. In such an operation it is preferred to employ asteam vulcanizer with a steam lock intermediate its length, whereby thecovered wire is first exposed to steam under a pressure of between 10and 250 p. s. i. and then to steam at a lower pressure after passingthrough the steam lock.

The following example illustrates the application of the presentinvention to the fabrication of commercial products.

A compounded siloxane was employed which was prepared by milling partsof titanla, 4 parts of benzoyl peroxide and 100 parts of a polymerproduced by polymerizing a mixture of low molecular weight cyclicdimethyl siloxanes with potassium hydroxide in the proportion of onepotassium per 500 silicons, the polymer being an insoluble solid of thenature of unvulcanized natural gum rubber. This compounded siloxane wasextruded in the cross-sectional shape desired for a gasket. Theextrusions were dusted with tale, and placed in a tray. A steamautoclave was brought up to temperature, and all condensate was blownout of the lines; The pressure was dropped sufilciently to open theautoclave. The extrusions were placed in the autoclave, immediately. Theautoclave was closed and the pressure immediately brought up. The timefrom placing the extrusions in the hot autoclave to pressuring of theautoclave was less than one p. s. i. for five minutes. The exhaust portsof the autoclave were opened and the vulcanized extrusions then removedand placed in an air oven. The oven was held at F. for two hours. .at

280 1'. for one hour and at 390 l". for tour 'atai'emperature higherthan the steam tempera.- hours. ture during vulcanization for atleast-one hour. I claim: 3. The method defined in claim 2 in whichthe 1. The method of producing organosilicon elastomers which comprisesvulcanizing an extrudable compounded siloxane composed of a filler,benzoyl peroxide, and a readily deformable dimethyl polysiloxane byheating the mixture in direct contact with saturated steam at a gaugepressure between 10 and 250 pounds per square inch for a period between2 and 1000 minutes,

said period being correlated with steam pressure to lie above the lineAB and below the line CD in the accompanying drawing, and thereaftercuring the vulcanized elastomer at a temperature higher than the steamtemperature'during vulcanizing for at least one hour.

2. The method of producing organosiloxane elastomers which comprisesvulcanizing an extrudable compounded siloxane which is composed oi areadily deformable organopolysiloxane which has a degree of substitutionbetween 1.75 and 2.25

mouovalent hydrocarbon =8adicals per silicon atom and which contains atleast 40 mol percent oi siloxane ,units of the type RRSiO in which Rrepresents alkyl and R represents a monovalent hydrocarbon radical treeof aliphatic unsaturaand 250 pounds per square inch for a period between2 and 1000 minutes, said periodbeing'correlated with steam pressure tolie above the line AB and below the line CD in the accompanying drawingand thereafter curing the vulcanized elastomer at substantiallyatmospheric pressure period from exposure of the siloxane to atemperature above 80 C. to the time the steam gauge pressure is up to atleast 10 pounds per square inch is less than 2 minutes.

4. The method defined in claim 2 in which the steam pressure isgradually reduced.

5. The method defined in claim 2 in which the initial temperature in thecuring step is below the temperature maintained during vulcanization,and the final temperature is higher than that maintained duringvulcanization.

6. The method in accordance with claim 2 wherein the vulcanizing agentis a diorganoperoxide in which at least one organic radical is anaromatic acyl radical.

EARL LEATHEN WARRICK.

lameness crrnn The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS The Vanderbilt Rubber Handbook. DD. 31, 266.

' 267, 8th edition. 1942.

Servais, Rubber Age. vol. .58. No. 5, February 1946, pp. 579 to 584.Ellis. The Chemistry of Synthetic Resins, vol.

1, page 286, Reinhold, 1985.

